Just to add a little more information. You can set the inital pose of the robot by publishing to the "initialpose" topic used by AMCL and documented here: http://www.ros.org/wiki/amcl#Subscribed_Topics. This is, in fact, what RVIZ does when you use the "2D Pose Estimate" button. You can always user "rostopic echo initialpose" to see exactly what it's sending.